Intelligent power integrated circuits, referred to as "power ICs," have become increasingly popular for implementing system functions with improved performance, reduced size and lower cost. The automotive industry in particular has made effective use of power ICs to offer improved features in such areas as more efficient engine management systems, safety systems, and comfort features. Power ICs typically have current capabilities ranging from a few milliamps to fifty amps and typically operate at voltages up to 60 volts or higher.
The output transistor of a power IC is typically a metal oxide semiconductor device, referred to as a "MOS" device. MOS devices have improved over the years and now have very low resistance when turned on, referred to as the "drain to source on-resistance," or "R.sub.dson." R.sub.dson may be as low as 0.4 ohms, or less.
In many applications, such as in automotive uses, the wires connected to the power IC may be accidentally shorted to a current source for brief periods of time. When this happens, the current flow may be limited only by R.sub.dson of the output transistor. Low values of R.sub.dson may allow excessively large currents to flow through the output transistor that may damage or destroy the output transistor.
Techniques have been developed to limit the output current so that the output device is not destroyed, and to allow normal operation to resume when the short circuit condition has been removed. One technique is to detect the temperature of the output transistor, or the entire power IC, and infer that a short circuit condition exists if the temperature exceeds a predetermined limit. The output device may be turned on and off periodically until the temperature is reduced. However, as R.sub.dson becomes lower, the current flowing during even brief periods may be large enough to damage the output device or related components. Also, turning on and off a large current may result in undesirable inductive voltage spikes due to inductive loads or length of the wiring harness. Furthermore, temperature sensing may result in other outputs on a multiple output power IC being turned off when they did not need to be.
Another technique is to sense the current flowing through each output device individually and to use an analog feedback loop to reduce the short circuit current to a level that will not damage the output transistor. Once the short circuit condition is removed, the current returns to a normal lower value and the power IC resumes normal operation. A circuit to carry out such a technique is illustrated in prior art FIG. 1. In normal operation, an external load is connected to output terminal 102. Power MOS device 110 acts as a switch to ground, thereby allowing current to flow from an external current source, through the load, and through MOS device 110 to ground. When MOS device 110 is turned off, no current flows through the load.
To sense a short circuit condition, a resistor R10 may be inserted in series with the drain of MOS device 110 and a voltage drop across resistor R10 measured. However, resistor R10 may waste a significant amount of power during normal operation. An improved sensing means is also illustrated in prior art FIG. 1. AMOS device and a resistor may be added in parallel with MOS device 110. Sense MOS device 112 and sense resistor R1 are connected as shown in prior art FIG. 1. The current flowing through resistor R1 is determined by the ratio of the series resistance of resistor R1 and R.sub.dson of sense MOS device 112 in parallel with R.sub.dson of output device 110. As drain current through MOS device 110 increases, the voltage drop across sense resistor R1 will increase proportionally. This voltage is applied to the gate of MOS device 114, which forms a feedback control loop. If the drain current through MOS device 110 becomes excessive, as sensed by resistor R1, MOS device 114 begins to turn on. Turning on MOS device 114 reduces the gate to source voltage, referred to as "V.sub.gs," that is applied to the gate of MOS device 110, thereby reducing the drain current.
Unfortunately, sense MOS device 112 is debiased by sense resistor R1 and the effective ratio decreases as the feedback pulls down on the gate of output device 110 and sense device 112. This causes a chopping phenomenon on the output as the feedback loop continually compensates for the sense MOS device ratio changing. For low inductive loads, this loop may become unstable.
Accordingly, it is an object of the invention to limit the drain current flowing through the output device of a power IC so that the output device is not damaged.
Another object of the invention is to overcome the problem of instability inherent in a feedback control loop.
Other objects and advantages will be apparent to those of ordinary skill in the art having reference to the following figures and specification.